Rigid and semi-rigid containers made from corrugated paperboard and thermoplastic materials are available in the market place in a variety of sizes. These containers, with or without interior thermoplastic liners or exterior coatings or laminates, offer the packager a container which can withstand the weight of many products, accommodate the large volume of many products, and often provide stackability where necessary for shipping or storage.
Examples of patents directed to containers include U.S. Pat. No. 3,927,821 (Dunning) which discloses a paper board carton having a bottom panel, and side and end walls, with a lining film of plastic adhered to the inner surface of the paper board.
British Patent Specification 1,136,885 (Anderson) discloses a container having an outer casing of comparatively stiff sheet material such as cardboard and an unbroken protective layer of thermoplastic material disposed in the casing. The casing comprises a bottom and a number of upright side walls.
Some containers include horizontal flanges extending either towards the interior of the container or towards the exterior of the container. Examples of patents directed to such containers include U.S. Pat. No. 2,660,362 (Schilling) disclosing a carton with horizontal flaps at each end thereof, and a cover sheet heat sealed to the flaps.
U.S. Pat. No. 3,866,816 (Bemiss) discloses a self supporting outer paper board tray and a plastic liner, as well as laterally extending flanges around the upper edge of the paper board or plastic liner.
U.S. Pat. No. 4,533,065 (Chazal et al) discloses container having a cardboard portion, portions of synthetic material, and an externally extending flange around the upper edge of the container.
Corrugated board containers internally lined with a thermoplastic liner are useful in many packaging applications, including food packaging.
Conventional corrugated paperboard includes two outer liner boards, and an intermediate fluted board located between the two outer liner boards.
Beneficial features of the board (rigidity and strength) are combined with the oxygen and moisture barrier properties of polymeric film. The thermoplastic liner should adhere to the inside surface of the corrugated board container with sufficient adhesion to prevent separation of the liner from the board surface during packaging, storage distribution and display.
When making a container such as that disclosed in U.S. Pat. No. 4,722,474 (Dropsy), using equipment such as that disclosed in U.S. Pat. No. 4,862,671 (Lanoiselee et al) adhesion of the liner to the board can be affected by many factors such as temperature settings on the machine, the physical and chemical composition of the liner, the nature of the product packaged inside the container, the manner of distribution and storage of the filled container, ambient conditions, etc.
The dimensions of the container are also important, since a container such as that disclosed in the Dropsy patent requires a thermoformable liner material which is required to form a deep pocket in the corrugated board shell.
When using the containers just described for packaging some meat and cheese items, it is often advantageous to use modified atmosphere packaging (MAP) techniques well known in the art. MAP can extend the shelf life of some food items. The effectiveness of MAP packaging depends in part on being able to maintain the physical integrity of the package, and avoid the occurrence of pinholes or other breaks in the package materials.
The inventor has discovered that the forming webs of the present invention offer particularly good adhesion to corrugated board, and also provide effective gas barrier for gases such as carbon dioxide, oxygen, or nitrogen used in modified atmosphere packaging methods.
It is therefore an object of the present invention to provide a forming web, for use as a liner in a semirigid or substantially rigid container, which exhibits good adhesion to the interior surfaces of the container.
It is also an object of the present invention to provide a forming web, for use as a liner in a semirigid or substantially rigid container, which exhibits good barrier properties with respect to gases typically used in modified atmosphere packaging, as well as gases, especially oxygen, in the ambient environment on the outside of the finished container.
It is yet another object of the present invention to provide a forming web, for use as a liner in a semirigid or substantially rigid container, which exhibits resistance to flex or stress cracks.
Another object of the present invention to provide a forming web, for use as a liner in a semirigid or substantially rigid container, which exhibits very low shrink and/or memory to prevent delamination of the inventive forming web from the box.
Still another object of the present invention to provide a forming web, for use as a liner in a semirigid or substantially rigid container, which exhibits deep drawability, i.e. the property of the web which allows it to be thermoformed into a relatively deep container.